Field of the Invention
The invention relates to a system and a method for providing sound to at least one user, wherein audio signals from an audio signal source, such as a microphone for capturing a speaker's voice, are transmitted via a wireless link to a receiver unit, such as an audio receiver for a hearing aid, from where the audio signals are supplied to means for stimulating the hearing of the user, such as a hearing aid loudspeaker.
Description of Related Art
Typically, wireless microphones are used by teachers teaching hearing impaired persons in a classroom (wherein the audio signals captured by the wireless microphone of the teacher are transmitted to a plurality of receiver units worn by the hearing impaired persons listening to the teacher) or in cases where several persons are speaking to a hearing impaired person (for example, in a professional meeting, wherein each speaker is provided with a wireless microphone and with the receiver units of the hearing impaired person receiving audio signals from all wireless microphones). Another example is audio tour guiding, wherein the guide uses a wireless microphone.
Another typical application of wireless audio systems is the case in which the transmission unit is designed as an assistive listening device. In this case, the transmission unit may include a wireless microphone for capturing ambient sound, in particular from a speaker close to the user, and/or a gateway to an external audio device, such as a mobile phone; here the transmission unit usually only serves to supply wireless audio signals to the receiver unit(s) worn by the user.
Typically, the wireless audio link is an FM (frequency modulation) radio link operating in the 200 MHz frequency band. Examples of analog wireless FM systems, particularly suited for school applications, are described in European Patent Application EP 1 864 320 A1 and corresponding U.S. Pat. No. 7,648,919 B2 and in International Patent Application Publication WO 2008/138365 A1 and corresponding U.S. Pat. No. 8,345,900 B2.
In recent systems, analog FM transmission technology has been replaced by technology employing digital modulation techniques for audio signal transmission, most of them working on other frequency bands than the former 200 MHz band.
U.S. Pat. No. 8,019,386 B2 relates to a hearing assistance system comprised of a plurality of wireless microphones worn by different speakers and a receiver unit worn at a loop around a listener's neck, with the sound being generated by a headphone connected to the receiver unit, wherein the audio signals are transmitted from the microphones to the receiver unit by using a spread spectrum digital signals. The receiver unit controls the transmission of data, and it also controls the pre-amplification gain level applied in each transmission unit by sending respective control signals via the wireless link.
International Patent Application Publication WO 2008/098590 A1 and corresponding U.S. Patent Application Publication 2010/019836 A1 relate to a hearing assistance system comprising a transmission unit having at least two spaced apart microphones, wherein a separate audio signal channel is dedicated to each microphone, and wherein at least one of the two receiver units worn by the user at the two ears is able to receive both channels and to perform audio signal processing at ear level, such as acoustic beam forming, by taking into account both channels.
In wireless digital sound transmission systems, not only audio data is to be transmitted but also control data, for example, for controlling the volume of playback of audio signals, for configuring the operation mode of the devices, for querying the battery status of the devices, etc. The transmission of such control data causes, compared to audio data transmission alone, overhead to the system in current consumption and/or delay which should be minimized.
There are certain known methods for concurrent transmission of audio data and control data. A schematic overview concerning the basic types of such concurrent transmission is shown in FIGS. 11A to 11D.
In general, transmission of control data can be made either “out-of-band” or “in-band”. In this context “out-of-band” means that different logical communication channels are used for audio data transmission and control data transmission, i.e., audio and control data are transmitted in separate digital streams. Such technique is used, for example, in mobile and fixed telephony networks. “In-band” means that control data is somehow combined with the audio data for transmission. In digital transmission of audio signals, usually the audio data as provided by the analog-to-digital converter is compressed prior to transmission by using an appropriate audio-codec. The resulting compressed audio data stream can be either transmitted sample-by-sample, i.e., as an essentially continuous stream, or in packets of samples.
FIG. 11D shows one way to control how data can be inserted in an in-band manner into a sample-by-sample transmitted audio stream. In the example shown in FIG. 11D control information is added to or mixed with the audio signal stream 52 prior to compression, wherein the control information may be represented by audible DTMF signals (see, for example, ITU recommendation G.23), or the control information may be inserted into the audio band by using inaudible spread spectrum techniques (see, for example, U.S. Pat. No. 7,844,292 B2). The mixture 49 of control information and audio information then undergoes compression prior to being transmitted.
Another known example of in-band control data transmission for sample-by-sample audio transmission is shown in FIG. 11A, wherein control data bits are interleaved with audio data bits in the compressed audio data stream, thereby forming a combined data stream 55. For example, the least significant one or two audio bits per octet may be substituted by control data bits, see for example, ITU recommendations G.722, G.725 and H.221, which standards are used in telephony networks.
A similar principle of in-band control data transmission for a packet-based audio data transmission is shown in FIG. 11B, wherein in each audio data packet a control field is reserved for transmitting control data together with audio data in a common packet 55A, 55B, 55C, see for example, International Patent Application Publication WO 2007/045081 A1 and corresponding U.S. Patent Application Publication 2007/0086601 A1 which relate to wireless audio signal transmission from a wireless microphone to a plurality of hearing instruments.
In FIG. 11C, an example of an out-of-band control data transmission is shown, wherein control data is transmitted as dedicated control data packets 50 which are separate from the audio data packets 51A, 51B, 51C. An example of such data transmission is described in U.S. Pat. No. 8,266,311 B2. Such a method is also used in the Bluetooth standard for headset profile, where control data is transmitted in different time slots (using ACL links) than those allocated for audio data (using SCO links).
Any such combined audio and control data transmission method either introduces a large delay in the transmission of the control commands or introduces a large overhead in terms of bit rate reserved for control traffic, which translates into a power consumption overhead.